Alcohol metabolism
Ethanol, an alcohol found in nature and in alcoholic drinks, is metabolized through a complex catabolic metabolic pathway.
Research suggest that animals evolved the ability to metabolize the alcohol present in fermented fruit to adapt to the changing climate 10 million years ago. Thanks to enzymes in their gut, and particularly one called ADH4, they can make use of the calories in alcohol.
The average human digestive system produces approximately 3g of ethanol per day merely through fermentation of its contents. Catabolic degradation of ethanol is thus essential to life, not only of humans, but of almost all living organisms. In fact, certain amino acid sequences in the enzymes used to oxidize ethanol are conserved all the way back to single cell bacteria. Such a functionality is needed because all organisms actually produce alcohol in small amounts by several pathways, primarily along the fatty acid synthesis, glycerolipid metabolism, and bile acid biosynthesis pathways. If the body had no mechanism for catabolizing the alcohols, they would build up in the body and become toxic. This could be an evolutionary rationale for alcohol catabolism also by sulfotransferase.
As is a basic organizing theme in biological systems, greater complexity of a body system, such as tissues and organs allows for greater specificity of function. This occurs for the processing of ethanol in the human body. All the enzymes needed to accomplish the oxidation reactions are confined to certain tissues. In particular, much higher concentration of such enzymes are found in the kidneys and in the liver, making such organs the primary site for alcohol catabolism. Variations in genes influence alcohol metabolism and drinking behavior.
The reaction from ethanol to carbon dioxide and water is a complex one that proceeds in three steps. Below, the Gibbs free energy of formation for each step is shown with ΔGf values given in the CRC.
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